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Anticoagulation in Atrial Fibrillation Yousif Ahmad1 and Gregory YH Lip2 1. Honorary Research Fellow; 2. Professor of Cardiovascular Medicine, University of Birmingham Centre for Cardiovascular Sciences, City Hospital, Birmingham, UK
Abstract Patients with atrial fibrillation (AF) are at increased thromboembolic risk, and they suffer more severe strokes with worse outcomes. Most thromboembolic complications of AF are eminently preventable with oral anticoagulation, and the increasing numbers of AF patients mean antithrombotic therapy is the most crucial management aspect of this common arrhythmia. Despite the proven efficacy of warfarin, a string of limitations have meant that it is underused by physicians and patients alike. This has prompted a search for new anticoagulants that could overcome many of the inconveniences of dose variability and anticoagulant monitoring associated with warfarin, but without sacrificing efficacy in thromboprophylaxis. The arrival of new oral anticoagulants has been complemented by improved risk stratification schemes, which permit clinicians to easily and reliably identify patients requiring anticoagulation and their bleeding risk. These advances in AF treatment will hopefully translate into improved outcomes for patients, especially as our experience with the new agents grows.
Keywords Atrial fibrillation, oral anticoagulation, stroke prevention Disclosure: Yousif Ahmad has no conflits of interest to declare. Gregory YH Lip has served as a consultant for Bayer, Astellas, Merck, AstraZeneca, Sanofi, BMS/Pfizer, Daiichi-Sankyo, Biotronik, Portola and Boehringer Ingelheim; he has been on the speakers bureau for Bayer, BMS/Pfizer, Boehringer Ingelheim and Sanofi Aventis. Received: 22 April 2012 Accepted: 17 May 2012 Citation: Arrhythmia & Electrophysiology Review 2012;1:12–6 Correspondence: Gregory YH Lip, University of Birmingham Centre for Cardiovascular Sciences, City Hospital, Birmingham, B18 7QH, England, UK. E: [email protected]
Support: The publication of this article was supported by Daiichi Sankyo Europe GmbH. The views and opinions expressed are those of the authors and not necessarily those of Daiichi Sankyo Europe GmbH.
Atrial fibrillation (AF) leads to a prothrombotic state1 and places patients at risk of thromboembolic disease. The most common and serious complication of thromboembolism is stroke, and AF is held responsible for 25 % of all strokes.2 Strokes in the context of AF are associated with a higher mortality, longer hospital stay and lower levels of independence at discharge.3 These factors combine to make antithrombotic therapy the most important management consideration for AF patients. AF is a growing clinical problem owing to the ageing population, and the advances in treatment and improved survival of patients with cardiac disorders such as ischaemic heart disease and heart failure, which predispose to AF.4 It is estimated that, by 2050, up to 16 million people in the US will suffer from AF,5 which is already the commonest arrhythmia encountered in clinical practice.6 The healthcare costs are substantial and AF accounts for 1 % of the National Health Service expenditure in the UK.7 Most thromboembolic complications associated with AF can be prevented with anticoagulant therapy.8 For over fifty years, the only effective therapeutic option to prevent stroke in AF patients were vitamin K antagonists – i.e., warfarin. Although warfarin has been proved to be highly efficacious as thromboprophylaxis, its limitations have negatively affected physicians’ willingness to prescribe – and patients’ willingness to receive – this effective treatment.9 This variability in prescribing habits for thromboprophylaxis has seen large numbers of patients left unprotected against stroke or inadequately protected with antiplatelet therapy,10 despite evidence highlighting that aspirin is an inferior option and carries its own substantial bleeding risk.11 The last decade has therefore seen an
intensive search for novel oral anticoagulant drugs that could overcome the limitations associated with warfarin and see more patients with AF properly protected against stroke. In addition, there has been advancement in the risk stratification of patients with AF. Although AF increases the risk of stroke fivefold,12 this risk is not homogenous and depends on the presence or absence of specific stroke risk factors.13 These risk factors were incorporated into the simple CHADS2 score14 and used to artificially categorise patients as ‘low’, ’intermediate’ or ‘high’ risk. Traditionally, guidelines have recommended oral anticoagulation for high-risk patients, aspirin for low-risk patients, and either anticoagulation or aspirin for the intermediate-risk group. The CHADS2 score also classifies many patients as ‘intermediate-risk’,15 and there is evidence that warfarin is superior to aspirin for this group of patients.16 Similarly, the evidence suggests that aspirin is ineffective as stroke prevention for patients identified as ‘low-risk’ by CHADS2.17 Therefore the utilisation of the CHADS2 score in isolation would result in substantial numbers of patients at risk of stroke being undertreated. These limitations of the CHADS2 score prompted efforts to establish a new schema that would be more inclusive of common stroke risk factors and would reliably identify ‘truly low-risk’ patients who do not require antithrombotic therapy, as well as reduce the number of patients at risk of stroke being denied oral anticoagulation. The CHA2DS2-VASc score18 has been proposed and, in various independent validation cohorts, has been shown to reliably identify
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‘truly low-risk’ patients (annual stroke rate 160 mmHg. One point is awarded for abnormal renal function and another point for abnormal liver function, and the same applies to drugs and/or alcohol. A score of 0–2 indicates a low risk of bleeding; a score ≥3 indicates a high risk of bleeding. Many of the risk factors for stroke are also risk factors for bleeding and, at higher HAS-BLED scores, the net clinical benefit of warfarin is actually greater. This is because there is a greater and more dramatic reduction in the risk of ischaemic stroke and a comparatively smaller increase in the risk of bleeding when warfarin is given to patients with high HAS-BLED scores. Patients with CHA2DS2-VASc scores of 0 are truly low-risk and derive a negative net clinical benefit from warfarin.25
Warfarin The efficacy of warfarin for preventing stroke and systemic embolism in AF patients was demonstrated in a string of randomised controlled trials (RCTs) over the preceding three decades, with a two-thirds risk reduction in ischaemic stroke compared with placebo. These data led to an increase in the use of warfarin and a consequent decrease in the number of strokes.26 As well as having a clear benefit over placebo, warfarin was also demonstrated to be superior to aspirin for stroke prevention in non-valvular AF.27 An RCT dedicated to evaluating stroke prevention in elderly patients with AF also showed the superiority of warfarin for the prevention of stroke, with no difference between warfarin and aspirin for major bleeding or intracranial haemorrhage.28 The benefit of warfarin is dependent on the amount of time that patients spend with their INRs in therapeutic range (TTR, time in therapeutic range).29 The optimal INR for patients with AF on warfarin for protection against thromboembolism has been established as 2.0–3.0, with an increased risk of thromboembolic and bleeding complications outside of this range. An increased TTR is associated with less thromboembolism, less bleeding, fewer myocardial infarctions and fewer deaths.30,31 Small improvements in TTR translate
ARRHYTHMIA & ELECTROPHYSIOLOGY REVIEW
Table 1: The CHA 2 DS 2 -VASc Score for Risk of Stroke in Non-valvular Atrial Fribillation Risk Factor
Congestive heart failure
Sex category: female
The name of the score is derived from the first letter of each risk factor. TIA = transient ischaemic attack.
Table 2: The HAS-BLED Score for Risk of Bleeding Risk Factor
Abnormal renal/liver function
1 or 2
Bleeding history or predisposition
Elderly (age over 65)
1 or 2
The name of the score is derived from the first letter of each risk factor. INR = international normalized ratio.
Table 3: Limitations of Warfarin •
Frequent monitoring necessitating regular clinic attendance
Narrow therapeutic window
Slow onset and offset of action, requiring 3–6 days to reach therapeutic levels
Numerous drug and dietary interactions
Genetic polymorphisms exist that confer increased sensitivity or resistance to warfarin
into significant benefits,32 with a low TTR potentially obliterating the benefit of anticoagulation. Self-monitoring can improve the quality of INR control33 and may bring the TTR closer to that achieved in clinical trials.34 Despite its unequivocal efficacy when properly used, warfarin has well documented limitations (see Table 3).35 Alternatives to warfarin must be proven to reliably perform at least as well as warfarin in RCTs, with an acceptable safety profile (ximelegatran was withdrawn due to hepatotoxicity36). New drugs should circumvent many of the limitations associated with warfarin that necessitate regular coagulation monitoring. They should therefore feature fixed-dose regimens, oral formulations, wide therapeutic windows, low propensity for food and drug interactions, and predictable pharmacokinetics and pharmacodynamics with little inter- and intra-patient variability.
Antiplatelet Therapy In 2006, the ACTIVE-W (Atrial fibrillation clopidogrel trial with irbesartan for prevention of vascular events) compared dual antiplatelet therapy with aspirin and clopidogrel to warfarin for the prevention of thromboembolism in AF.37 The trial was stopped early due to the clear superiority of warfarin over dual antiplatelet therapy. Furthermore, the
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Clinical Arrhythmias Table 4: Pharmacokinetic and Pharmacodynamic Properties of Novel Anticoagulants Dabigatran
Mechanism of action
Direct thrombin inhibitor
Direct factor Xa inhibitor
Direct factor Xa inhibitor
17 hours with multiple doses,
9 hours in healthy subjects,
7–9 hours with single doses
12 hours in elderly subjects
80 % cleared renally
One-third cleared renally, two-thirds
70 % cleared in faeces,
metabolised by the liver
25 % cleared renally
Mode of excretion Age effect
Affects pharmacokinetic parameters
Interaction with aspirin at high doses
Tmax = time to maximum plasma concentration.
rates of bleeding in the two groups were very similar (2.4 % per year for clopidogrel and aspirin versus 2.2 % per year for warfarin). The evidence proves conclusively that antiplatelet therapy is an inferior option when compared with warfarin for stroke prevention. The comparable rates of bleeding also mean that, in patients deemed at too great a haemorrhagic risk, oral anticoagulation would not be a suitable treatment option.
Novel Oral Anticoagulants There have been concerted efforts to unearth new drugs that would fulfil the criteria for the ideal anticoagulant and could represent a viable alternative to warfarin. The three new drugs currently available fall into two classes: oral direct thrombin inhibitors (dabigatran) and oral factor Xa inhibitors (rivaroxaban and apixaban).38 Their pharmacokinetic and pharmacodynamic properties are summarised in Table 4.
Dabigatran Dabigatran is a competitive, direct and reversible inhibitor of thrombin. The RE-LY (Randomized evaluation of long term anticoagulant therapy with dabigatran etexilate) trial was a large RCT comparing dabigatran with warfarin. 39 This Phase III, blinded, non-inferiority trial studied 18,113 patients, either with non-valvular AF with a CHADS2 score of 1 or higher, or aged over 65 years with coronary artery disease. Patients were randomised to either dabigatran at a dosage of 110 or 150 mg twice daily or warfarin (INR of 2–3). The primary efficacy outcomes were stroke or systemic embolism. They occurred at a rate of 1.69 % per year in patients assigned to warfarin, versus 1.53 % per year in patients assigned to dabigatran 110 mg (relative risk [RR] 0.91, 95 % confidence interval [CI] 0.74–1.11, p